Modify response headers only if response has not yet been sent

If an endpoint works async the response may already been sent to the
client while requests are still being send to the SPARQL endpoint. Only
modify the response headers, based on the info received from the SPARQL
endpoint, if it has not yet been sent to the client.

Author: erikap

helpers/mu/sparql.js

@@ -33,8 +33,8 @@ function newSparqlClient() {
 function query( queryString ) {
   console.log(queryString);
   return newSparqlClient().query(queryString).executeRaw().then(response => {
-
-    if (httpContext.get('response')) {
+    const temp = httpContext;
+    if (httpContext.get('response') && !httpContext.get('response').headersSent) {
       // set mu-auth-allowed-groups on outgoing response
       const allowedGroups = response.headers['mu-auth-allowed-groups'];
       if (allowedGroups) {